The present invention relates to data communications and more particularly to an FM radio communication system and method that efficiently employs the SCA band of system FM stations for wide area transmission and reception of digital data.
In the past, the public phone system has provided cost effective business and other data communication where the receivers are located at known fixed locations. However, wiring cannot be used for data communication to mobile receivers, and the public phone system is thus not able to provide data communication for mobile receivers.
Radio transmission has been employed with the use of especially dedicated frequencies to transmit business or other data such as paging data or stock market quotes to mobile and fixed receivers. However, serious disadvantages have been imposed on the dedicated frequency approach to data communications as a result of limited availability of dedicated frequencies and high capital and operating costs. Further, it has not been economically feasible for low volume or infrequent users to employ a dedicated frequency for data communication.
In the United States, FM radio stations are granted a license for a range of frequencies that may be used for the main FM radio signal but which is substantially larger than the minimum range required for the main FM radio signal. Historically, the excess bandwidth has not been used for purposes other than transmitting the main FM radio signal, but radio stations are now free to lease frequencies in the excess bandwidth to other users through a service known as Subsidiary Communications Authorization ("SCA"). The FCC has deregulated SCA service and stations are free to carry SCA services without prior authorization, so long as all uses of the frequency are within the regulations imposed on the license holder.
The FM radio baseband signal has an authorized range of 0 to 99 KHz. The portion of the baseband used for primary FM radio covers the frequency range 0 to 53 KHz, and the SCA band is the portion of the baseband signal that lies in the range from 53 KHz to 99 KHz. Generally, the percentage deviation of the FM signal that results from the SCA must not exceed 20% (assuming that the basic FM programming is stereophonic) with 10% being attributable to the portion of the SCA signal in the range from 53 to 75 KHz and 10% from the portion of the SCA signal in the 75 KHz to 99 KHz range.
There is also a regulatory limit on the amount of out-of-band emissions from the SCA signals that fall in the passband of the basic program signal. When the SCA baseband is added to the primary FM radio baseband, the entire baseband is referred to as a "composite" baseband.
Since an FM station transmits basic FM radio programming in the 0 to 53 kHz range, the station may lease all or a portion of the unused SCA bandwidth 53 to 99 KHz to other users without a requirement for licensing the lessee(s). Historically, the unused frequency range has been divided into three parts and a user has leased one or more of those parts depending on user requirements. Typical current uses of FM-SCA include paging, transmission of stock market information, background music, and traffic reporting. A special receiver is necessary to extract the SCA service from the composite signal.
The FM-SCA band has also been employed by FM stations to transmit business or other data, such as paging system data. Prior FM-SCA data transmission has generally been based on a frequency assignment system and each FM-SCA user accordingly has been limited to an assigned narrow band signal within the SCA band A typical assigned band is 2 to 3 KHz wide. Typical examples of prior frequency divided FM-SCA transmitting schemes are set forth in: U.S. Pat. No. 4,379,947, entitled "System for Transmitting Data simultaneously With Audio" and issued to Paul Warner on Apr. 12., 1983, and U.S. Pat. No. 5,146,612 entitled "Technique for Using a Subcarrier Frequency of a Radio Station to Transmit, Receive and Display A Message Together With Audio Reproduction of the Radio Program", and issued to Grosjean et al., on Sep. 8, 1992.
In prior paging systems, phone number data, and possibly some message data, is transmitted over a wide area through regional FM stations with the use of a narrow SCA frequency band at each FM station. Once transmitted data is received by a mobile receiver, a beep is generated by the receiver and a return call or other required action may be undertaken.
As in the case of the dedicated frequency approach, the transmission of paging or other data through an assigned SCA frequency has had limited user availability and has been costly. It has generally been inefficient since at times the transmission system is largely unused, while, at peak traffic times, capacity constraints delay messages. Moreover, the SCA assigned frequency approach has generally incurred costly administrative overhead required for multiple lease setups.
While data communication between computers has been provided with cost effectiveness through the public telephone network, little development has occurred in the potential use of FM radio for intercomputer data communication, especially where at least one of the communicating computers is a mobile (or portable) computer. As a result, business, government and other organizations have had limited capability for communicating inventory information, work order information, and other financial or work related data especially to recipients having a portable computer.
A mobile cellular phone with a modem can establish mobile data communications by phone system connection to another phone having a modem. However, such data communication is point-to-point and requires that a phone connection be provided.
Additionally, in the cellular telephone industry, significant effort has been expended to make roaming more convenient. However, in the absence of appropriate data communication capabilities, calls generally cannot be placed to a roamer if the location of the roamer's cellular phone is unknown.
Stolen vehicle identification systems represent another specific prior art area where limited data communication capability has limited the effectiveness of system operation. A hidden transmitter in a stolen car must be activated to mark the car location. As long as the car is in the area covered by the identification system, a locally transmitted high frequency signal can be used to activate the hidden transmitter in a stolen car. However, with limited prior art data communication capability, once a car has been moved to an area outside the range of the identification system, the hidden transmitter may be activated only if a costly system is employed to send activation signals throughout the nation or other wide geographic area.
There are undoubtedly many other potential systems or applications involving wide area mobile receivers that have remained undeveloped in the absence of prior art data communication capabilities that might only be available, if at all, through costly capital and/or operating expenditures.
In a copending patent application Ser. No. 07/737,407, entitled "COMMUNICATIONS SYSTEM", filed by Irl Benham and David A. Wright on Jul. 29, 1991, assigned to World Broadcasting Development, Inc., which is an affiliate of the present assignee, and herein incorporated by reference, there is disclosed a new multi-channel transmission system for national or other wide area transmission through time shared, wide band use of the SCA band of regional FM radio stations (hereinafter referred to as the wide band FM-SCA radio system).
In operation, the wide band FM-SCA radio system uses essentially the entire available FM-SCA spectrum as a single "wide band" channel. Information signals to be transmitted are divided into segments, fractions of a second long. The information segments are coded, and audio information is compressed prior to transmission. The signal segmentation allows a variety of different signals to be transmitted simultaneously over the FM-SCA spectrum through use of time sharing. A receiver responds to a code sent at the beginning of each signal segment and uses it to put the segments back together in the proper sequence. The entire process can have little or no noticeable effect on the sound quality of the audio content of the transmission.
The wide band FM-SCA radio system constructs a 0 -42 kHz waveform, and combines several techniques which allow much more information to be transmitted on the SCA band of each FM station than was previously available. This is accomplished by the use of single side band (SSB) modulation, use of the entire SCA bandwidth at once with time division multiplexing, and, in the case of audio transmission, use of time-compression. SSB modulation achieves twice the spectral efficiency of modulation containing two sidebands.
As indicated, a time-divided structure is used by the wide band FM-SCA radio system for sharing the SCA bandwidth among different users. Each user has access to the entire SCA channel for a fraction of the time, as opposed to frequency-division where each user has access to a portion of the spectrum all of the time. In the disclosed embodiment, time is divided into 140 millisecond frames, which correspond to and are synchronous with 2,660 cycles of the 19 kHz pilot tone. A small portion of the frame is devoted to digital control data that specifies the exact time assignments of the audio signals and their content.
The wide band FM-SCA radio system has extensive information transmitting capacity. The referenced copending application discloses a basic system structured for the transmission of audio information.
As indicated above, prior art transmission of data information to mobile receivers has been characterized with disadvantages that have limited the availability and use of such data transmission. A basic need has thus existed for wide area or national transmission of data information to receivers that may be mobile or fixed on an efficient and cost effective basis. The wide band FM-SCA radio system can accommodate multiple users simultaneously and is thus well suited for meeting the need for economic transmission of data information to mobile receivers over a wide or national geographic area.